Echo and Soft VoIP PBX Systems

The new world of Internet telephony is facing one of the same challenges that early long-distance calling did. Here's one of the techniques for doing a high-quality call over VoIP.

Most of us have experienced telephone calls with disturbing echoes on
the line. Low echo volumes together with discernible delay
can make a line completely unusable, with the call being terminated after
the exchange of a few halting sentences. Traditionally, problems with
echo have been experienced on long-distance or international calls,
particularly those involving satellite connections.

For many people new to software-based VoIP telephony systems, such
as Asterisk, the phenomenon of voice echo comes as an unpleasant
surprise. This is true even for those who come to the business
after working with traditional PBX systems or proprietary VoIP
equipment. Suddenly echo is a problem on local calls, and the
traditionally troublesome long-distance and satellite calls are
completely echo-free.

In this article, we discuss the origins of echo and how it manifests
itself in the VoIP world with particular reference to Asterisk and other
software-based telephony systems.

Where Does Echo Come from and Why Is It a Problem?

Echo in telephony systems is caused by two main
phenomena: the first is electrical echo due to
imperfect impedance matching, and the second is
acoustic echo due to microphone pickup of audio
output. Both these sources produce similar effects and
have to be treated similarly. The major difference
is electrical echo is a property of the
line connection and remains mostly constant
throughout the call, while acoustic echo varies in
strength and delay depending on the changing acoustic
environment of the echo source. For instance, on a
hands-free cell-phone call, the echo characteristics
change as the speaker moves around.

Electrical signals of all types always are reflected at line
terminations,
except when the load at the line end exactly matches the impedance rating
of the line itself. In fact, the meaning of, say, “75-ohm
cabling” is
precisely that in order to have no signal reflections, the cable must
be terminated by a 75-ohm load. Line impedance is a property of
the cable that is affected only by the cable geometry. As no cables are
geometrically perfect over their length and no load impedance is perfectly
accurate, there always is some reflection at a line termination.

Where digital signals are concerned, as long as the reflections are a
small enough fraction of the data transmission, the reflections do not
cause errors in reading the bit values. Thus, digital systems can tolerate
considerable echo.

The human ear has quite different characteristics, however; it is an incredibly
sensitive instrument. The softest sound that can be heard has an acoustic
power about a hundred thousand billion times smaller than the
power at the threshold of pain. As long as sounds vary by
only about
a factor of 100 or so, the ear hears a similar level of sound.
So even
what electrically looks like a small reflection can sound about the same
volume as the original signal to the human ear.

And, the traditional telephone circuits are far from perfect. Two-wire
circuits from analog lines terminate at devices called hybrids
that convert the two-wire analog signal to four-wire signals before
digitization. The loads at the hybrids vary quite widely, as does the
impedance of the low-cost subscriber loop wiring.
The result is
almost every call that involves an analog telephone anywhere in the
circuit has electrical reflections that can be interpreted by the
ear as troublesome echoes.

If this is so, why is echo not a problem on every call? The answer
is, if the echo is heard at the same time as the caller is speaking,
it is heard as part of the side tone and goes unnoticed. Echo becomes
noticeable only when there is a delay between speaking and hearing your voice
echoed. This is why echo is a problem only for traditional telephony
over
long distances. The round-trip delay on a coast-to-coast US call is more
than
30ms, which is enough for echo to cause irritation. Satellite delays
are much longer still.

VoIP intrinsically has packetization, depacketization and processing
delays built into its protocols. That is why, from the point of view of
echo, every VoIP call is like a very long-distance call.

Figure 1 shows a typical VoIP scenario. The echo is heard on
the VoIP phone: the caller on the analog line hears only a normal side tone,
because there are no signal delays. Because delay is a necessary component
of perceived echo, traditional PBXes that switch analog or T1/E1 traffic
have no perceived echo problems, as their intrinsic end-to-end delay is
low. It is the packetization and processing delays inherent in VoIP that
cause existing echo to become a problem.

An Engineer at digium claimed that echo training does in fact work on a pri and, from my limited experience enabling echotraining on a pri, I would tend to believe him. How do you think that would work?

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